The present invention relates to solderless bonding of component leads to pads located on a non-rigid substrate. Specifically, the invention relates to solderless bonding of fine-lead TAB components to pads disposed on an epoxy printed circuit board and, more specifically, relates to the use of a combination of laser energy and ultrasonic energy to bond leads to bare copper pads.
Electronic card assembly has recently evolved from Plated-Thru-Hole to Surface Mounted Technology. The current state of the art in low input/output component (50 mil lead pitch) attachment to an epoxy (FR-4) printed circuit board is solder paste screen printing on the pads, followed by placement of the component, and solder reflow. Components having 25 mil lead spacing, such as flat-packs, are also assembled and attached selectively using a Pick-Place-Attach tool which solders the leads on pads previously covered with solder.
Another technique is gang bonding where thermodes apply a combination of pressure and heat to achieve simultaneous thermocompression bonding of the leads to the pads. While gang bonding provides high throughput rates, the technique has long been plagued with problems such as chip cracking when inner-lead bonding and defective solder joints when outer-lead bonding. Single-point interconnection is currently emerging as a viable alternative to simultaneous or gang bonding of leads to pads for both inner-lead bond and outer-lead bonding.
Tape-Automated-Bonding (TAB) packages with fine leads are being developed for high function electronic products. In an attempt to reduce the problems associated with gang bonding, an alternative technique, Single-Point-TAB (SPT) bonding has been developed where the bonding is performed sequentially using a fine-point tip and heating the substrate to a temperature above the threshold temperature for bonding.
A major problem encountered when using Single-Point-TA bonding for outer-lead bonding is the heating limitations of the epoxy substrate. Therefore, heating of the bonding tip is a more practical approach for SPT bonding. While electrically heated tips are limited to slow throughput bonding operations, it has been shown that laser-heated tips are capable of providing bonding at high throughput rates.
Solderless lead-to-pad outer lead bonding for attaching fine pitch components to epoxy substrates has been attempted using conventional thermosonic techniques in which the entire substrate is heated and bonding is performed seriatim, one lead at a time, using a fine bonding tip energized with ultrasonic pulses. Such efforts generally are unsuccessful as a result of poor coupling of the ultrasonic energy to the epoxy substrate. Generally, good ultrasonic energy coupling requires that the pads be located on a rigid substrate. In order to compensate for the inherent poor coupling, higher heating temperatures have been tried with negative results since the temperature may reach a level where heat damage occurs to the substrate.
The use of laser energy for bonding wires to pads is well known in the art. For example, in IBM Technical Disclosure Bulletin, Vol. 29, No. 11, April 1987 entitled "Intrinsic--Thermocouple Process Monitor" a continuous laser beam of short duration is delivered via an optical fiber to heat a tungsten bonding tip which, in turn, melts a gold-coated copper wire and bonds the wire to an EC pad located on an aluminum or glass ceramic rigid substrate. The input laser power is controlled by feeding back a signal commensurate with the wire temperature during bonding. A similar device is described in an article entitled "Discrete Wire Bonding Using Laser Energy" by P. Chalco et al, Semiconductor International, May 1988. Solderless bonding of copper leads to copper pads using laser energy is described in an article entitled "Diffusion Bonding", Research Disclosure, No. 273, January 1987. The combined use of laser and ultrasonic energy for bonding surfaces together is described in U.S. Pat. No. 4,534,811 issued Aug. 13, 1985, entitled "Apparatus For Thermo Bonding Surfaces" and assigned to the same assignee as the present application. A non-contact laser/ultrasonic welding technique where ultrasonic energy is used for cleaning the weld joint is described in U.S. Pat. No. 4,330,699, issued May 18, 1982 and entitled "Laser/Ultrasonic Welding Technique."
In each of the above-mentioned prior art references there is an absence of bonding a lead to a pad located on a resilient, non-rigid substrate, such as an epoxy printed circuit board. The ability to bond leads directly onto bare-copper pads located on epoxy printed circuit boards is an important aspect of the present invention.
The present invention overcomes the above-described problems associated with solderless outer-lead bonding of leads to organic substrates, such as FR-4 epoxy, by bonding gold-coated leads to bare-copper pads using a laser-assisted thermocompression technique where the laser energy is used to pulse heat a fine-point capillary tip. The tip is placed in forced intimate contact with a superposed lead and pad.
A pulse of ultrasonic energy is applied to the heated tip to achieve a diffusion bond at the interface of the lead and pad.
Bond reliability is greatly improved as a result of the absence of solder and the fact that only three parameters control the bonding process. The three parameters are tip pressure, laser power and bonding time. In order to achieve bonding rates comparable to that achieved when gang bonding, a highly controllable laser heating pulse and a fast x-y stepper such as is currently in use in wirebonding applications are combined to achieve bonding rates in the order of 50 to 100 milliseconds per bond.
The present invention also solves the problem of achieving reliable solderless bonds with bare-copper pads without resultant heat damage by bonding the lead to the pad using a combination of laser energy and ultrasonic energy. In prior methods, long duration (&gt;300 ms) laser-sonic pulses were used to bond wires to gold coated pads on rigid (ceramic or alumina) substrates. In contrast, the present invention concerns a bonding method for joining leads to bare-copper pads disposed on a non-rigid (epoxy) substrate using short duration (&lt;100 ms) laser-sonic pulses.
The tip used for bonding the leads to the pads contain a small diameter cavity in which the laser energy is reflected and absorbed for heating the tip. The frontal surface of the tip is composed a centrally disposed hole, forming the cavity, and two grooves intersecting at a substantially 90.degree. angle in the vicinity of the cavity. The four islands thus created impress a footprint into the lead in intimate contact with the tip. The footprint so produced provides an indication of the weld quality.
Another feature of the present invention resides in the ability to analyze the footprint impressed by a bonder tip on a lead bonded to a pad in order to determine bond quality. An intelligent fast inspection tool analyzes the footprint and provides a signal indicative of the acceptance or rejection of the bond before a subsequent bond is attempted.